I have installed WordPress on the Xammp server on my local machine. With a plugin Simply Static, I have hosted this WordPress site on GitHub pages.
Now I am going to access its admin page by wp-admin, I put my credentials and I am having an Error named 405 Not Allowed. The URL targeted for login is my GitHub pages' URL instead of the local machine.
How could I fix this issue or Login to my wp-admin page?
GitHub pages are designed for hosting static html only. They cannot execute PHP or any other server side scripting language, so regardless of this specific error you won't be able to interact with the admin console at all. The plugin you mentioned makes static copies of your entire site, so it will copy the admin URL but it won't be functional.
You need to access the admin page on your local, original wordpress environment instead.
If you wish to host your full site online, in interactive mode, find a webhost which supports PHP and specifically wordpress. There are hundreds of possibilities.